In the production of nitric acid, for example, ammonia is oxidized to nitrogen monoxide over platinum gauzes and in the subsequent operation is oxidized by (atmospheric) oxygen to form nitrogen dioxide. The nitrogen dioxide in the course of the operation is cooled down and absorbed in water in counter-current in an absorber, to form nitric acid.
For this procedure, ammonia is vaporized in an ammonia vaporizer and is mixed with process air upstream of the oxidation reactor. Because the ammonia used does not consist 100% of ammonia, but generally has a water fraction usually in the order of magnitude of around 0.2% to about 0.5%, water accumulates within the ammonia vaporizer. This results in a pressure drop in the ammonia vaporizer, thereby lowering the pressure needed for the inward feeding for the process air.
In order to increase the pressure in the ammonia vaporizer, the water must be removed. In accordance with the prior art, the water is removed by continuous or discontinuous draining/desludging of ammonia from the ammonia vaporizer into the ammonia stripper. As a consequence of this operation, not only water but also ammonia is drained/desludged into the ammonia stripper. The ammonia content of the liquid which is drained from the ammonia vaporizer into the ammonia stripper is generally between about 60% and about 100%. In the ammonia stripper, the ammonia is then stripped with steam from the water/ammonia mixture and is supplied to the operation. The ammonia stripper operates under virtually the same pressure as the ammonia vaporizer. This leaves a solution consisting of more than about 80% of water. The remainder is ammonia. After the stripping procedure, this liquid (water/ammonia mixture) is moved into an atmospheric “knock-out drum.” In the atmospheric knock-out drum, a portion of the ammonia is vaporized again, owing to the lower pressure level by comparison with the ammonia stripper, and is supplied via a chimney to the atmosphere. The water, which has a low ammonia content, is subsequently supplied from the knock-out drum to a disposal vessel or to a wastewater purification facility. The ammonia-containing water must be disposed of in accordance with the conditions for environmental protection. This is accomplished, for example, by supplying the disposal container to a refuse incineration plant.
Because the absorber in a nitric acid plant does not carry out 100% absorption of the nitrous gases, there are still nitrous gases in the residual gas from a nitric acid plant. These gases are generally supplied to a residual-gas cleaning facility. The residual-gas cleaning facility used is typically a DeNOx or an EnviNOx® system. The catalytic reaction for reducing the nitrous gases in the residual gas (DeNOx or EnviNOx®) requires ammonia. This is generally supplied, parallel to the process-side ammonia vaporization, to an ammonia vaporizer for residual-gas cleaning, where it is vaporized before being added to the residual-gas stream or the residual-gas cleaning facility.
Thus a need exists to provide a process for preparing nitric acid, having the features of the generic process specified above, in which ammonia-containing wastewater requiring removal from the system is no longer produced, or in which the amount of ammonia-containing wastewater is at least substantially reduced.